This invention relates in general to a solid waste disposal and resource recovery process which produces a useful fuel or chemical synthesis gas, and more particularly, to an improvement upon the process disclosed in U.S. Pat. No. 3,729,298, hereinafter referred to as the Anderson process.
Historically, the least expensive method for disposing of solid waste has been open dumping. However, unprocessed garbage dumps produce severe problems of ground water pollution through leaching, loss of land value, fire hazards and rodent infestation. A more acceptable method, sanitary landfilling has reduced these problems by composting and covering the garbage with earth. Nevertheless, in large metropolitan areas, this practice has become increasingly unattractive, as acceptable sites become more scarce. Both of these methods have been supplemented by incinerating the waste before landfilling. While conventional incineration provides significant reductions in the volume of the refuse and some alleviation of the pollution caused by leaching, it introduces new environmental problems such as air pollution, and though volume reductions of 80 to 90 percent are possible, the residue or ash is not biologically inactive and therefore landfilling is still required. Furthermore, resource recovery from conventional incineration tends to be minimal.
A highly desirable solution to the above problems is disclosed and claimed in U.S. Pat. No. 3,729,298; the disclosure of which is incorporated herein by reference. In summary, the Anderson process disclosed in said patent comprises feeding refuse into the top portion, and oxygen into the base of a vertical shaft furnace. The furnace (or converter) has three functional zones; a drying zone at the top, a thermal decomposition or pyrolysis zone in the middle, and a combustion and melting zone (or hearth) at the base. As the refuse slowly descends in the furnace, it is first dried by the hot gas which rises through the furnace, and then pyrolyzed. Pyrolysis is a process whereby organic matter in the refuse is thermally decomposed and cracked in an oxygen-deficient atmosphere, with the generation of char, oils and a gas containing a high concentration of CO and H.sub.2. As the refuse moves down through the pyrolysis zone, it is converted to volatile materials which rise and char which descends into the combustion zone. There the char is combusted with oxygen, causing the generation of carbon monoxide and carbon dioxide which produce the heat required to melt the inorganic solids in the refuse, such as glass and metal. The molten residue is continuously tapped from the furnace hearth and quenched in a water bath. A gas, consisting of at least 50% CO plus H.sub.2 (on a dry basis) is discharged from the top of the furnace. Following cleanup, the gas is ready for use as a medium BTU fuel gas or for chemical synthesis.
In the above-described Anderson process, oxygen is required in the furnace for two purposes. One is to gasify the char by oxidation, to form primarily carbon monoxide; and the second is to supply heat by combustion to satisfy the energy requirements of the process.
The oxygen containing gas required in the above-described Anderson process must contain at least 40% oxygen by volume in order that it supply sufficient energy to the hearth to melt the inorganic solids. Such a gas may be made by enriching air with oxygen. Oxygen concentrations greater than 40% may also be used, with commercial oxygen being the most preferred from a technical point of view. Economics will dictate the exact amount of oxygen, between the limits of 40 to 100%, to be used in a given situation, which will depend also on the composition and moisture content of the refuse.
As pointed out in detail in U.S. Pat. No. 3,729,298 (note particularly column 6, lines 23-42) Anderson found the weight ratio of oxygen to refuse fed into the shaft furnace to be an important parameter of his process. Specifically, Anderson found that his process could be operated very efficiently at low ratios of oxygen to refuse, namely, a weight ratio of oxygen to refuse entering the shaft furnace in the range of from about 0.15:1 to 0.28:1.
The advantages afforded by the Anderson process resulting from the use of low ratios of oxygen to refuse are numerous. Most obvious is the cost savings associated with low ratios of oxygen to refuse; namely, minimizing the cost of oxygen. Further advantages which result relate to the composition of combustible gas leaving the shaft reactor. Operating within the weight ratio of oxygen/refuse in the range of 0.15:1 to 0.28:1 in the reactor, viz, an amount substantially less than required for the stoichiometric combustion of the refuse, results in the production of a gas having a high concentration of combustible constituents such as carbon monoxide and hydrogen. Thus, the gas can be used as a chemical synthesis gas, such as, for example, in the synthesis of methanol or as a reducing gas in applications, such as the reduction of iron ore pellets to metallic iron. Alternatively, the fuel energy of the combustible gas can be utilized to advantage by burning the gas completely.